2026-08-03 – N-fragment counterpoise architecture

Goal

Generalize the ORCA counterpoise (BSSE) correction from two fragments/ neutral-singlet-only to N fragments with independent per-fragment charge, via a small engine-agnostic library (this package) plus a thin, generalized substep inside orca_step.

Contents:

• An N-fragment, charge-aware counterpoise architecture
  • Goal
  • The physics
  • Confirmed design detail: ORCA preserves full-cluster indexing
  • Architecture
  • The Compound-script path: kept as the N = 2 oracle, then retired
  • Out of scope for this phase
  • Sequence / milestones
  • Psi4 sub-step: IMPLEMENTED (2026-08-04)
  • Validation plan
  • Relationship to earlier documents
  • Open questions
• Summary
• Evidence
• Why this matters beyond Na/Cl
• Fix directions (not yet implemented)
• Raw data